Use of ultrasonically vibrating surgical blades to cut and coagulate tissue has previously been disclosed, for example, in U.S. Pat. No. 2,714,890. The benefits of using ultrasonic energy for tissue dissection and coagulation are many, e.g., enhanced cutting speed, simultaneous hemostasis and cutting, freedom from electrical hazards since no electrical current flows in the tissue, elimination of smoke and reduced build-up of eschar and other material on the blade. An ultrasonic surgical instrument adapted for laparoscopic usage under endoscopic observation has been described in prior U.S. patent application Ser. Nos. 07/670,186, filed Mar. 15, 1991, now abandoned and 07/828,697, filed Feb. 3, 1992, now abandoned, disclosures of which are incorporated herein by reference. From a review of those disclosures, it will be appreciated that there is an evolving use of ultrasonic instruments for laparoscopic surgery. In those instruments, a shaft diameter of approximately 10 mm or more was believed necessary for a number of reasons: (1) the ultrasonic energy transmission waveguide, i.e., a shaft, should have a sufficiently large diameter to provide the necessary strength to carry the desired ultrasonic energy without failure from fatigue; (2) there should be sufficient space for a protective sheath which surrounds the waveguide, i.e., the shaft, to prevent coupling of the ultrasonic energy to tissue or devices adjacent to it; and (3) the shaft should have a sufficiently large diameter to obtain the desired stiffness to allow the surgeon to supply side, as well as longitudinal, forces to the tissue.
While the ultrasonic laparoscopic instruments disclosed in the above-identified prior applications, of common assignee herewith, are eminently suitable for their intended purposes, there is the limitation that they require use of a 10 mm diameter trocar port which, in turn, creates a larger incision and scar than would be the case when using smaller available ports. Because most laparoscopic procedures involve the use of both 5 and 10 mm trocar ports, a 5 mm diameter device for laparoscopic surgery would enable the surgeon to choose from a greater variety of approaches to the surgical site because a 5 mm device can be used through either a 5 or 10 mm trocar port. There are, however, difficulties in reducing the diameter of an ultrasonic laparoscopic instrument for use with a 5 mm trocar port consistent with the above-identified criteria for effectively transmitting ultrasonic energy to the blade to enable dissection and coagulation. For example, in most prior ultrasonic devices which employ a sheath, it is required that the sheath be telescoped over the shaft from one end. Consequently, the sheath must have a larger diameter than the outside diameter of the shaft and at least one of the blade and shaft coupling at its proximal end to the handpiece. Where the sheath inner diameter is smaller than the outside diameter of both the blade and the shaft coupling to the ultrasonic transducer, the acoustic drive train, i.e., the shaft, must be formed into separable pieces to enable the user to remove the sheath. In this connection, see U.S. Pat. No. 3,861,391.